CHAPTER 8
PHOTOSYNTHESIS
The Pathways of Photosynthesis
(continued)
4. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer
look
5. Alternative mechanisms of carbon fixation have evolved in hot, arid
climates
6. Photosynthesis is the biosphere’s metabolic foundation: a review
4. The Calvin cycle uses ATP and NADPH
to convert CO2 to sugar: a closer look
• The Calvin cycle regenerates its starting material
after molecules enter and leave the cycle.
• CO2 enters the cycle and leaves as sugar.
• The cycle spends the energy of ATP and the reducing
power of electrons carried by NADPH to make the
sugar.
• The actual sugar product of the Calvin cycle is not
glucose, but a three-carbon sugar, glyceraldehyde-3phosphate (G3P).
• Each turn of the Calvin cycle fixes one carbon.
• For the net synthesis of one G3P molecule, the
cycle must take place three times, fixing three
molecules of CO2.
• To make one glucose molecules would require six
cycles and the fixation of six CO2 molecules.
• The Calvin cycle has three phases.
• In the carbon fixation phase, each CO2 molecule is
attached to a five-carbon sugar, ribulose
bisphosphate (RuBP).
• This is catalyzed by RuBP carboxylase or rubisco.
• The six-carbon intermediate splits in half to form two
molecules of 3-phosphoglycerate per CO2.
• During reduction, each 3-phosphoglycerate
receives another phosphate group from ATP to
form 1,3 bisphosphoglycerate.
• A pair of electrons from NADPH reduces each 1,3
bisphosphoglycerate to G3P.
• The electrons reduce a carboxyl group to a carbonyl
group.
• If our goal was to produce one G3P net, we would
start with 3 CO2 (3C) and three RuBP (15C).
• After fixation and reduction we would have six
molecules of G3P (18C).
• One of these six G3P (3C) is a net gain of carbohydrate.
• This molecule can exit the cycle to be used by the
plant cell.
• The other five (15C) must remain in the cycle to
regenerate three RuBP.
• In the last phase, regeneration of the CO2 acceptor
(RuBP), these five G3P molecules are rearranged
to form 3 RuBP molecules.
• To do this, the cycle must spend three more
molecules of ATP (one per RuBP) to complete the
cycle and prepare for the next.
• For the net synthesis of one G3P molecule, the
Calvin recycle consumes nine ATP and six
NAPDH.
• It “costs” three ATP and two NADPH per CO2.
• The G3P from the Calvin cycle is the starting
material for metabolic pathways that synthesize
other organic compounds, including glucose and
other carbohydrates.
5. Alternative mechanisms of carbon
fixation have evolved in hot, arid climates
• One of the major problems facing terrestrial plants is
dehydration.
• At times, solutions to this problem conflict with other
metabolic processes, especially photosynthesis.
• The stomata are not only the major route for gas
exchange (CO2 in and O2 out), but also for the
evaporative loss of water.
• On hot, dry days plants close the stomata to conserve
water, but this causes problems for photosynthesis.
• In most plants (C3 plants) initial fixation of CO2
occurs via rubisco and results in a three-carbon
compound, 3-phosphoglycerate.
• These plants include rice, wheat, and soybeans.
• When their stomata are closed on a hot, dry day,
CO2 levels drop as CO2 is consumed in the Calvin
cycle.
• At the same time, O2 levels rise as the light reaction
converts light to chemical energy.
• While rubisco normally accepts CO2, when the
O2/CO2 ratio increases (on a hot, dry day with
closed stomata), rubisco can add O2 to RuBP.
• When rubisco adds O2 to RuBP, RuBP splits into a
three-carbon piece and a two-carbon piece in a
process called photorespiration.
• The two-carbon fragment is exported from the
chloroplast and degraded to CO2 by mitochondria and
peroxisomes.
• Unlike normal respiration, this process produces no
ATP, nor additional organic molecules.
• Photorespiration decreases photosynthetic output
by siphoning organic material from the Calvin
cycle.
• A hypothesis for the existence of photorespiraton (a
inexact requirement for CO2 versus O2 by rubisco) is
that it is evolutionary baggage.
• When rubisco first evolved, the atmosphere had far
less O2 and more CO2 than it does today.
• The inability of the active site of rubisco to exclude O2
would have made little difference.
• Today it does make a difference.
• Photorespiration can drain away as much as 50% of the
carbon fixed by the Calvin cycle on a hot, dry day.
• Certain plant species have evolved alternate modes
of carbon fixation to minimize photorespiration.
• The C4 plants fix CO2 first in a four-carbon
compound.
• Several thousand plants, including sugercane and corn,
use this pathway.
• In C4 plants, mesophyll cells incorporate CO2 into
organic molecules.
• The key enzyme, phosphoenolpyruvate carboxylase,
adds CO2 to phosphoenolpyruvate (PEP) to form
oxaloacetetate.
• PEP carboxylase has a very high affinity for CO2 and
can fix CO2 efficiently when rubisco cannot - on hot,
dry days with the stomata closed.
• The mesophyll cells pump these four-carbon
compounds into bundle-sheath cells.
• The bundle sheath cells strip a carbon, as CO2, from the
four-carbon compound and return the three-carbon
remainder to the mesophyll cells.
• The bundle sheath cells then uses rubisco to start the
Calvin cycle with an abundant supply of CO2.
• In effect, the mesophyll cells pump CO2 into the
bundle sheath cells, keeping CO2 levels high
enough for rubisco to accept CO2 and not O2.
• C4 photosynthesis minimizes photorespiration and
enhances sugar production.
• C4 plants thrive in hot regions with intense
sunlight.
• A second strategy to minimize photorespiration is
found in succulent plants, cacti, pineapples, and
several other plant families.
• These plants, known as CAM plants for crassulacean
acid metabolism (CAM), open stomata during the
night and close them during the day.
• Temperatures are typically lower at night and
humidity is higher.
• During the night, these plants fix CO2 into a variety of
organic acids in mesophyll cells.
• During the day, the light reactions supply ATP and
NADPH to the Calvin cycle and CO2 is released from
the organic acids.
• Both C4 and CAM plants add CO2 into organic
intermediates before it enters the Calvin cycle.
• In C4 plants, carbon fixation and the Calvin cycle are
spatially separated.
• In CAM plants, carbon fixation and the Calvin cycle are
temporally separated.
• Both eventually use the Calvin cycle to incorporate
light energy into the production of sugar.
6. Photosynthesis is the biosphere’s
metabolic foundation: a review
• In photosynthesis, the energy that enters the
chloroplasts as sunlight becomes stored as chemical
energy in
organic
compounds.
• Sugar made in the chloroplasts supplies the entire
plant with chemical energy and carbon skeletons to
synthesize all the major organic molecules of cells.
• About 50% of the organic material is consumed as fuel
for cellular respiration in plant mitochondria.
• Carbohydrate in the form of the disaccharide sucrose
travels via the veins to nonphotosynthetic cells.
• There, it provides fuel for respiration and the raw
materials for anabolic pathways including synthesis of
proteins and lipids and building the extracellular
polysaccharide cellulose.
• Plants also store excess sugar by synthesizing
starch.
• Some is stored as starch in chloroplasts or in storage
cells in roots, tubers, seeds, and fruits.
• Heterotrophs, including humans, may completely or
partially consume plants for fuel and raw materials.
• On a global scale, photosynthesis is the most
important process to the welfare of life on Earth.
• Each year photosynthesis synthesizes 160 billion metric
tons of carbohydrate per year.